The vapor phase polymerization method provides a common process for producing a polyolefin such as polyethylene, in which an olefin monomer such as ethylene is polymerized through a vapor phase reaction in the presence of, for example, a titaniferous solid catalyst or metallocene catalyst.
In this vapor phase polymerization method, for example, referring to FIG. 5, a solid catalyst A is fed through a supply line 12 into a fluidized bed reactor 10 and, simultaneously, a gaseous olefin is caused to pass through a supply line 13 and blown thereinto from a bottom of the fluidized bed reactor 10 through a gas distributor plate 11. The gas distributor plate 11 is composed of, for example, a porous plate which has a plurality of through holes, and is arranged in the vicinity of the bottom of the fluidized bed reactor 10. In this way, a fluidized bed (reaction system) 14 is formed and held in the fluid state in the fluidized bed reactor 10, and whereby a polymerization reaction is carried out in the fluidized bed 14. Polymer particles produced by the polymerization reaction in the fluidized bed 14 are continuously discharged through a line 15 from the fluidized bed reactor 10. For example, unreacted gaseous olefin having passed through the fluidized bed 14 of the fluidized bed reactor 10 has its flow rate reduced in a velocity reduction zone 16 provided in an upper part of the fluidized bed reactor 10 and is discharged outside the fluidized bed reactor 10 through a gas outlet 10A disposed at a top of the fluidized bed reactor 10. The unreacted gaseous olefin having been discharged from the fluidized bed reactor 10 is caused to pass through a recycling line 17 and blown into the fluidized bed 14 of the fluidized bed reactor 10. The above gaseous olefin is continuously supplied through a supply line 20 which is combined with the recycling line 17.
The recycling gas such as unreacted gaseous olefin having been discharged from the fluidized bed reactor 10 must be passed through a heat exchanger (cooler) prior to the re-blowing into the fluidized bed 14 of the fluidized bed reactor 10 because of the need of being deprived of heat of polymerization (i.e. heat generated by the polymerization reaction). When this cooler is disposed upstream of a gas recycling device, namely, between the gas outlet of the fluidized bed reactor and the gas recycling device, a condensate of gaseous monomer such as gaseous olefin containing polymer powder which has been generated by cooling by the cooler is fed in misty form to the gas recycling device such as blower (or compressor). As a result, a gas recycling pipe is clogged, and such mist is entrained to the distributor plate and gas recycling pipe of the polymerizer to thereby cause clogging and other serious trouble. Accordingly, in the prior art, it is a common practice to arrange the cooler 19 downstream of the gas recycling device such as blower 18, namely, between the gas recycling device such as blower 18 and the supply line 13 as shown in FIG. 5 in order to not only avoid the above trouble but also enhance the heat exchange efficiency.
However, the arrangement of the above cooler 19 involves the problem that, in accordance with the increase of the amount of recycled gas and the increase of the temperature thereof, the size of the blower must be large, the pipe size must be large and heat insulation promoting equipment and highly heat resistant seal are required with the result that the recycling equipment per se becomes huge.
Blowers for transporting a gas or conducting a cyclone treatment thereof are widely used in chemical and petroleum plants. The blowers are mainly divided into turboblowers in which an impeller is rotated in a gas so that the velocity and pressure of gas passing through the blower are increased by the action of the blade and displacement blowers in which the volume of a gas sealed in a predetermined volume is reduced and the pressure thereof is increased with the use of back pressure. The turboblowers are subdivided into centrifugal blowers in which a gas passes in a radial direction in an impeller and a pressure increase is attained by the centrifugal action of the impeller and axial blowers in which a gas passes in an axial direction in an impeller and a pressure increase is attained by the action of blade lift. The displacement blowers include rotary blowers in which the gas sucked by the rotation of the rotor provided in a casing is pressurized by reducing the volume defined by the inner wall of the casing and the rotor by the rotation of the rotor. The rotary blowers include two-lobe type blowers (Roots blowers) in which two two-lobe rotors are mounted in a differentiated phase in a casing so as to be rotatable without contacting each other and rotated in directions opposite to each other by means of a timing gear to thereby pressurize a gas so that the gas is transported under pressure.
With respect to these blowers, it is a common practice to provide a slide part or rotational slide part positioned between the discharge side thereof as a high pressure side and the suction side thereof as a low pressure side with a contact-type gas leakproof seal such as mechanical seal or a noncontact-type gas leakproof seal such as labyrinth seal, carbon ring seal or oil film seal so that the gas leakage can be prevented (gas edge cut). In particular, the turboblower has a labyrinth seal provided at a rotational slide part positioned between the impeller connected to its main shaft and the suction port, while, in the Roots blower, the gas leakage is prevented by regulating the clearance present between the periphery of each rotor and the inner wall of the casing and the clearance present between the rotors.
Although the labyrinth seal clearance or above rotor clearance varies depending on the type and capacity of the blower in the use of the noncontact type seal, it is generally set at about 0.5 mm in order to minimize the gas leakage and enhance the blower efficiency (for example, compression efficiency).
However, when, for example, a polyolefin powder containing gas is treated with the use of the above blower for recycling a powder containing unreacted gas or conducting a cyclone treatment thereof in the vapor phase polymerization comprising subjecting an olefin monomer such as ethylene to a vapor phase reaction to thereby obtain a polyolefin such as polyethylene, it may occur that a frictional force is generated between the powder and the labyrinth seal, impeller, rotor or inner wall of casing at the time of passing of the powder through the gas leakproof seal clearance and rotor. As a result, the powder passing through the gas leakproof seal clearance and rotor clearance suffers from heat buildup and melts to thereby form stringy melt polymer, which is entrained to the distributor plate and gas recycling pipe of the polymerizer to thereby cause clogging and other serious trouble.
In the vapor phase polymerization in which, as mentioned above, the blower is employed in the transfer of a powder containing gas under pressure or the increase of the pressure thereof for the circulation and cyclone treatment of unreacted gas, powder crushing, deformation and integration may occur at the gas leakproof seal part clearance or rotor clearance between the high pressure part and the low pressure part. As a result, powder having suffered from crushing, deformation and integration remains in the gas leakproof seal part clearance or rotor clearance to thereby cause heat buildup, so that the life of the blower per se is caused to be short. Further, fine particles are formed, so that removal of such fine particles must be included in the after treatment to thereby complicate the process. Further, the above combined powder occasionally unfavorably causes clogging of, for example, gas recycling pipes.
Enlarging the gas leakproof seal part clearance or rotor clearance so as to allow powder to pass therethrough without heat buildup can be contemplated as means for solving the above problem. This is, however, disadvantageous in that an extreme gas leakage occurs at the gas leakproof seal part and the blower suffers from an efficiency drop with the result that plant operating cost is increased to an economic disadvantage.